[Bug 1624320] Re: systemd-resolved appends 127.0.0.53 to resolv.conf alongside existing entries

[Vincent Fortier]

>From the man page systemd-resolve can run in 3 mode of operations.

1) Ubuntu 17.10 default - The default is to list the 127.0.0.53 DNS stub
(see above) as only DNS server. This file may be symlinked from
/etc/resolv.conf in order to connect all local clients that bypass local
DNS APIs to systemd-resolved. This mode of operation is recommended.

2) WHAT YOU WANT: systemd-resolved maintains the
/run/systemd/resolve/resolv.conf file for compatibility with traditional
Linux programs. This file may be symlinked from /etc/resolv.conf and is
always kept up-to-date, containing information about all known DNS
servers....

3) PRE-17.04: Alternatively, /etc/resolv.conf may be managed by other
packages, in which case systemd-resolved will read it for DNS
configuration data. In this mode of operation systemd-resolved is
consumer rather than provider of this configuration file.

The fix:
root at localhost:~# ls -la /etc/resolv.conf 
lrwxrwxrwx 1 root root 29 mar  7 20:20 /etc/resolv.conf -> ../run/resolvconf/resolv.conf
root at localhost:~# rm -f /etc/resolv.conf
root at localhost:~# ln -s /run/systemd/resolve/resolv.conf /etc/resolv.conf
root at localhost:~# ls -la /etc/resolv.conf 
lrwxrwxrwx 1 root root 32 mar  8 07:30 /etc/resolv.conf -> /run/systemd/resolve/resolv.conf

Finally firefox started working properly along with all my command line
tools...

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https://bugs.launchpad.net/bugs/1624320

Title:
  systemd-resolved appends 127.0.0.53 to resolv.conf alongside existing
  entries

Status in systemd package in Ubuntu:
  Incomplete

Bug description:
  systemd-resolved, or more precisely the hook script
  /lib/systemd/system/systemd-resolved.service.d/resolvconf.conf, causes
  resolvconf to add 127.0.0.53 to the set of nameservers in
  /etc/resolv.conf alongside the other nameservers.  That makes no sense
  because systemd-resolved sets up 127.0.0.53 as a proxy for those other
  nameservers.  The effect is similar to bug 1624071 but for
  applications doing their own DNS lookups.  It breaks any DNSSEC
  validation that systemd-resolved tries to do; applications will
  failover to the other nameservers, bypassing validation failures.  And
  it makes failing queries take twice as long.

  /etc/resolv.conf should have only 127.0.0.53 when systemd-resolved is
  active.

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